The last time that neither player was available, the Chelsea manager was playing his last league game for the club 12 years ago.
"We have had big games before [but] this is certainly a test for us," he told the club's official website. "They are two influential players and we are going to miss them.
"I think all the players will step up and show that we are a team with character, with personality.
"Games against Manchester United were always very competitive and quite balanced, enjoyable games you looked forward to."
Di Matteo went on to praise the form of Oscar and Eden Hazard ahead of the match, which kicks off at 4pm this afternoon.